Agitator bead mills, also referred to as attritors, are known with a vertically or horizontally disposed agitator shaft.
In wet operation of corresponding agitator bead mills known in the art, a flowable grist suspension is conveyed, in particular continuously by means of a pump, in a housing of the agitator bead mill, from a product inlet on the input side of a grinding chamber, to a product outlet on the output side of the grinding chamber.
The agitator shaft of a grinding or agitator apparatus, which can also contain agitating elements such as rods or disks or can be simply a smooth-walled agitator body, feeds energy into a grinder media filling (not shown) in the grinding chamber. The impacting and thus the breakdown of the grist occurs at the contact points of the grinding media moved by rotation of the agitator shaft, in particular by impact and shearing forces. A solid content of 30% to 60% is customary in the suspension. This corresponds to a load of 300 kg to 600 kg of grist per m3 of liquid, such as water in particular.
Before the product emerges, a separation system, consisting in particular of a rotating outer basket and a static cylindrical sieve, is disposed in its interior. Separation of grinding media remaining in the mill and of the suspension with milled grist that thus leaves the bead mill occurs in the separation system.
In principle, a mill of this type can also be operated dry, as is familiar in the art. In order to ensure similar conditions in the grinding chamber, especially with respect to particle concentration, however, only a very low gas volume stream (as opposed to liquid, in wet operation) can be applied with the grist, said stream not being sufficient in itself for product transport from the product input all the way through the product output. The cause is the lower density of gases, by about a factor of 1,000 in comparison with liquids.
In the grinding chamber itself, this usually causes no problems since the movement of the agitator shaft and grinding media also “carries along” the grist. What is problematic, on the other hand, is the product conveyance in the area of the separation system. At this site, there can be sticking and crust of the grist. These stickings and crust grow “from behind” into the grinding chamber. There is a breakdown in the removal of grist and thus the flow rate of the agitator bead mill that remained dry. This results, finally, in the disadvantage that continuous operation is then no longer possible.
As general background technology, an agitator mill, according to DE 44 32 200 C1, includes a mill container that encloses a grinding chamber, an agitator apparatus rotatably disposed in it concentrically to the center longitudinal axis of the mill container, a power motor coupled with the agitator apparatus, a line diverting grist and grinding media out of the grinding chamber, a device for separation of grist and auxiliary grinding auxiliary media that is connected with the output line and separated from the agitator mill, a grist feeder and auxiliary grinding medium recirculating line connected with the separation device and a grist feeder line on the one hand and with the grinding chamber on the other hand, and a grist pump into the grist feeder line. Here the separation device comprises a housing in which a rotor is disposed that can be rotary-powered independently of the power drive of the agitator and can advance the auxiliary grinding media centrifugally from the grist and into the line that feeds the grist and circulates the auxiliary grinding media. The mill container, the output line for grist and auxiliary grinding media, the separation device and the line feeding the grist and circulating the grinding auxiliary media form a closed system. Only the grist pump disposed in the grist feeder line serves as a device to convey grist and grinding auxiliary media in the line feeding grist and circulating auxiliary grinding media.
Similarly, patent WO 2012/055388 A2 discloses as general prior art an agitator bead mill for grinding dry or non-dry substances, with a mill container that is filled at least partly with auxiliary grinding media, an inlet and an outlet for the grindable material or grist, a sieve disposed in an outlet area, an agitator shaft running through the center of the mill container, and several grinder elements disposed on the agitator shaft. Here, a first cage is linked to an inlet area and a second cage to the outlet area. A fluid inlet is linked to the inlet area. A cleaning apparatus is disposed in the center of the sieve.
Patent DE 10 2007 054 885 A1 relates to a method for fractioning of a dispersion of oxidic nanoparticles. Here a wet grinding process is described, which uses a membrane crosswise flow filtration, such that overflowing of the membrane is generated with dispersion by power-driven rotating parts.
U.S. Pat. No. 5,967,432 A relates to fully explicitly straight mills, which do not work with beads as grinding media and in which, instead, the material is crushed by collision and friction within itself and with respect to the inner surface of the grinding chamber. Technical features in connection with the use of grinding media basically cannot be taken from this publication.
Patent 2 595 117 A discloses a method for grinding in a vertical mill with grinding media with a continual removal of grinding media and grist at the upper end and an external separation apparatus. In the separation apparatus, “over-size material fractions” and grinding media are selected out by an upwardly flowing air stream in order to be fed back into the mill.
From patent DE 42 02 101 A1, a method for treating dry to damp material is known, such that the material is conveyed onward by a transported assortment of grinding media by gravitational action. Material conveyed by the aggregate of grinding media undergoes comminution, for example, during its passage. A fluid is added to this fractioned material shortly before its exit from the aggregate or comminution process and is mixed with it. With respect to the apparatus, this publication discloses a vertical grinding mill with a cylindrical, vertical-axis mill container inside which an agitator screw and grinding media are situated. A material intake is disposed in the upper area of the mill container and a material removal in the base. Closely above the material removal, nozzles are disposed radially in the wall of the mill container and inclined in the direction toward the material removal. The axes of the nozzles intersect with the axis of the mill container. Shortly before and/or during the product removal, a fluid is to be added by means of the nozzles. They serve to keep the motion of the fluid in the direction of the material transport. Contrary thereto, our method causes the fluid to move in a direction diverted away from the transport direction of the grist.